Acoustic wood for musical instruments (so called resonance wood) should be as light as possible but at the same time have a high modulus of elasticity (E-modulus or Young's modulus, respectively) and a high speed of sound. Moreover, it should be free of knots and have narrow, homogeneous annual rings and a low proportion of latewood (<20%). Only a few, carefully selected wood assortments meet these strict quality criteria.
Musical instruments which were built during the late 17th and early 18th centuries in many cases have better quality characteristics than contemporary instruments. One of the hypotheses for explaining this difference attributes the particular wood quality of these instruments to the climate situation known as the Maunder minimum, which prevailed between 1645 and 1715 and in which longer winters and colder summers evidently resulted in a slower and more uniform wood formation and thus provoked a smaller proportion of latewood. In the last decades of his work (the so-called “golden era”), the famous violin maker Antonio Stradivari mainly used spruce wood of trees that had grown during the Maunder minimum. These instruments have long been regarded as a sound ideal that has only rarely been achieved again.
The (acoustic) material quality of resonance wood is generally defined by the quotient c/p, wherein c is the speed of sound and p is the raw density of the resonance wood (Ono & Norimoto, 1983; 1984; Spycher, 2008; Spycher et al., 2008; Tab. 4). The speed of sound corresponds to the square root of the ratio of the E-modulus (for bending longitudinally to the fiber) to the density. The E-modulus is a material parameter which is independent of geometry; the product of E-modulus and area moment of inertia yields the flexural rigidity of the workpiece (Ono & Norimoto, 1983; 1984; Spycher, 2008; Spycher et al., 2008). The speed of sound of e.g. spruce wood in the longitudinal direction is 4800 to 6200 m/s, the average raw density is 320 to 420 kg/m3. Both parameters, like many other wood properties, depend on the moisture content of the wood, which increases the requirements regarding precision and infrastructure of the experiments, but also regarding the evaluation of test results. Of particular interest for all measures aiming to improve material quality is the impact that relative changes in modulus and raw density have on the speed of sound. If for a specific measure the E-modulus (in %) changes approximately proportionally to the change in raw density (in %), then the speed of sound will remain approximately the same (the material quality will then increase approximately inversely proportional to a reduction in raw density); such a ratio of relative changes in the E-modulus and raw density is called “narrow” (Ono & Norimoto, 1983; 1984; Spycher, 2008; Spycher et al., 2008). If, on the other hand, the E-modulus (in %) decreases significantly less than the raw density (in %), then the speed of sound will increase (the material quality will then increase more than inversely proportional to a reduction in raw density). Such a ratio of relative changes in the E-modulus and raw density is called “wide” or “large” and is highly desirable for achieving a high material quality of resonance wood (Schleske, 1998; Wegst, 2006). However, resonance wood with a wide E-modulus to raw density ratio is rarely found in nature and accordingly is expensive (Bond, 1976; Bucur, 2006).
Various methods for improving the acoustic properties of resonance wood have been tried. In particular, it has been proposed in EP 1734504 A1 to expose the resonance wood to the action of a wood-decomposing fungus species during a limited treatment time. In doing so, the fungus species and the duration of treatment should be chosen in such manner that, on the one hand, the treatment achieves an increase in the ratio between speed of sound of the wood and raw density of the wood and, on the other hand, strength values of the resonance wood do not fall below a predetermined minimum. Fungal species used were Asco- and Basidiomycetes from the family of Leotiaceae, Polyporaceae, Schizophyllaceae, Trichlomataceae and Xylariaceae. To perform the method, a feedboard method was used in which the resonance wood to be treated is placed between two fungus-infected woods with the same dimensions.
Subsequently, extensive investigations have shown that compared to the method according to EP 1734504 A1 a more pronounced improvement of the resonance wood would be desirable. In particular, it was found that none of the proposed fungus species is able to increase the damping factor of the resonance wood. An increase in the damping factor while simultaneously improving the acoustic material quality reduces the high tones of the instrument, which often sound painful to the listener.
In this regard, it has surprisingly been found that by means of a treatment with Physisporinus vitreus an improvement of the above mentioned acoustic material quality values while simultaneously increasing the damping factor can be achieved, whereby an overall improvement in the acoustic properties is obtained (Schwarze, F. W. M. R., Spycher, M., Fink, S. (2008) Superior wood for violins—wood decay fungi a substitute for cold climate. New Phytologist 179, 1095-1104).
A disadvantage of the methods described so far is that a uniform colonization of the wood can not be guaranteed by the selected fungus species. An irregular colonization has the consequence that the acoustic material quality is improved only inconsistently or not at all. Moreover, it entails the risk of undesirable strength losses, cracks and crevices in the wood. Moreover, it has been found that Physisporinus vitreus has a low level of competitivity with other fungus species and is, therefore, very susceptible to contamination by other species.
In the technical article Fuhr, M. J. et al. (2012) Automated quantification of the impact of the wood-decay fungus Physisporinus vitreus on the cell wall structure of Norway spruce by tomographic microscopy. Wood Sci Technol 46,769-779, there is described a method of automatic visualization and quantification of microscopic cell wall elements of spruce wood, which is also able to show the changes caused by Physisporinus vitreus. 
WO2012/056109 A2 describes the use of plant-derived nanofibrillated cellulose in the form of a hydrogel or a membrane as a carrier material for various types of cell cultures.